Page 50 - Photoreactive Organic Thin Films

PHOTOISOMERIZATION OFAZOBENZENES 29

1.5.13 TheTripet State
The energy of the triplet states of pseudo-stilbenes cannot be detected by
phosphorescence. In some cases, transient absorption or energy transfer gives
146
some information. Corner et al. investigated the temperature-dependent
kinetics of population and decay of the triplet state of 4-dimethylamino~4'-
nitroazo-benzene by laser flash experiments and found that its energy is
1 130
higher than 165 kj mole" . Monti et al. used the energy transfer method
3 1
and found a (ft,7t*) state at 140 kj mole" ; they inferred nonpianarity by
twisting of the azo bond ("phantom triplet state"). Higher triplet states may
147
play a role in biphoton holographic storage; this proposition, however, has
been challenged. 148
Because the thermal Z —» E isomerization is very fast, the Z-isomers
cannot be isolated. The spectra are available only from an extrapolation of
149
different E/Z mixtures produced by conventional flash irradiation and have
to be considered qualitative.

.5.2 Isomerization of Molecules of the Pseudo-StilbeneType
1.5.2. I Thermal Isomerization
There is a great difference between the two types of pseudo-stilbenes—
donor/acceptor-substituted and protonated azo compounds—with regard to
the stability of the Z-forms. The former isomerize quickly, whereas the latter
may be relatively stable.

L5.2.LI Protonated Azo Compounds
The rate of Z-azobenzene-H* isomerization is dependent on acid
138 140
strength: ' It is low below 70% of H 2SO 4 (by weight), the half lives at room
143
temperature at 83 wt% H 2SO 4 are 20 min, at 90% 7 min. This is rational-
2
ized by the formation of an intermediary ABH 2 * species in concentrated
acids, which indeed has been detected in SbF 5/FSO 3/SO 2 ("magic acid") at
-80°C. 150 Surprising are the high negative entropies of activation of the reac-
tion. Unfortunately, there are no studies of the temperature dependence of the
thermal isomerization of complexed azobenzenes. This type of compound
may be of higher importance in complex matrices than hitherto suspected.

f.5.2. f.2 Donor/Acceptor-Substituted Azo Compounds
Contrary to the Z-forms of protonated azobenzenes, the Z-forms of
donor/acceptor di-substituted azobenzenes isomerize very quickly at room
temperature. To investigate this reaction, the Z-form is produced by flash
excitation. The isomerization of these pseudo-stilbenes is strongly dependent
130 133 149 151
on the polarity of the solvent. ' ' ' For instance, 4-diethylamino-4'-
-1
nitroazobenzene isomerizes with k z_^ E = 0.007 s in hexane, but with k z^£ =
1
149 152
600 s" in N-methylformamide. ' 4-Anilino-4'-nitroazobenzene in cyclo-
3 153
hexane obeys the relation In k z_^ E = 22(*3) - 72(±9)-10 /(8.314«T).
154 155
Schanze et al. ' have established two linear relationships between the free-
activation enthalpy of isomerization and the Kosower Z values of aprotic and
156 157
protic solvents. Sanchez and de Rossi ' report a strong pH sensitivity of